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Katie Ledecky is competing only with herself, it seems, and Michael Phelps has to clear out more space in his medal case. Phelps, the most decorated athlete in Olympic history, added to his staggering haul Sunday night in the 4x100-meter freestyle relay, giving the US a lead it never relinquished, the AP reports. Afterward, he collected his 19th gold medal. "When I was on the block, I honestly thought my heart was going to explode out of my chest," Phelps said. "I was so hyped tonight and so excited." It was quite a night for the Americans, who were shut out on the golds on the opening night of swimming. Racing nothing but the clock, Ledecky gave the US its first victory by crushing her own world record in the 400 freestyle. She beat her nearest competitor by nearly 5 seconds.

The unassuming teenager from suburban Washington has dominated the longer freestyle events since winning gold in the 800 free at the London Olympics as a 15-year-old. When Ledecky saw the time—3:56.46—she let out an uncharacteristic scream and shook her right fist. Ledecky, who added gold to the silver she won in the women's 4x100 free relay, is also favored in other two individual events: the 200 and 800 free. She could pick up another gold in the 4x200 free relay. It was a night of world records in Rio. Britain's Adam Peaty set his second mark in as many nights in the 100 breaststroke, while Sarah Sjostrom of Sweden eclipsed her own mark in the 100 butterfly. In all, six world records have fallen on the first two days of swimming.

Michael Phelps competes in the final of the men's 4x100-meter freestyle relay. (David J. Phillip)